Method of manufacturing cylindrical ceramic capacitors



April 1969 R. J. COWLES 3,435,496

METHOD OF MANUFACTURING CYLINDRICAL CERAMIC CAPACITORS Original FiledOct. 11, 1963 EL E 9 5 INVENTOR. /.3 EaaEeTJo/w Cawu': i i

2o H15 Ar roeue'v United States Patent U.S. Cl. 29-25312 1 ClaimABSTRACT OF THE DISCLOSURE The method of manufacturing cylindricalceramic ca pacitors wherein a mixture of ceramic powder granules with atemporary binder are molded to form a cylindrical ceramic body havingaxial aligned recesses in each end thereof. The ceramic molded body isthen baked to vitrify the same. Electrodes are formed on the surfaces ofthe recesses by coating each of the recesses with a silver paste andthen the ceramic molded body with the molded electrodes is again bakedto fuse the coated electrodes to the ceramic molded body. A specialsolid cylindrical terminal head is formed having a spiral groove alongthe terminal head perimetral surface by upsetting, under compression,one end of a terminal wire. The terminal head is cemented to theelectrode in each of the recesses by applying a thermosetting conductivecement to the terminal head and curing the cement in situ by baking theassembled cylindrical ceramic capacitors to form a bond to eachelectrode and its respective terminal head.

This application is a division of patent application Ser. No. 315,602filed Oct. 11, 1963 now Patent No. 3,256,471 issued on June .14, 1966.

This invention relates generally to the method of manufacturing ceramiccapacitors and more particularly to the method of manufacturingcylindrical ceramic capacitors of the axial lead type.

The principal object of this invention is an improved method ofmanufacturing and producing tubular type ceramic capacitors possessinghighly desirable characteristics which is simple in construction and,thus, economical to manufacture.

In the method of manufacturing the cylindrical ceramic capacitorsillustrating the features of this invention, ceramic powders includemetallic oxides in various proportions and are molded into the desiredceramic body which is baked at a predetermined temperature in order tohold the blended materials in a vitrified homogeneous mass. The ceramicbody, as molded and baked, has cylindrical recesses in the ends thereofwithin which silver electrodes are flashed through the process of bakingat a predetermined temperature. After the flashing of the silverelectrodes in each of the recesses of the ceramic body, the terminalhead of each of of the terminals is cemented into the coated electroderecess and subsequently the ceramic capacitor, as assembled, is cured inorder that the terminal head is bonded securely to the silverelectrodes.

Thus, the method comprising this invention involves the steps of first,baking the cylindrical molded ceramic body which has a cylindricalrecess in its ends, in order that the body will be vitrified; second,flashing silver electrodes in each of the recesses; third, cementing aterminal head to each electrode in each of the recesses of the ceramicbody; and fourth, curing the cemented assembly.

The improved method of this invention is a capacitor which is much moredurable and mechanically rugged as compared to the thin walled tubulardisc type capacitors.

The use of solid type terminal heads reduces electrode inductance whichmakes this highly desirable for operation and also prevents burn spotsand holes and functions as a heat sink when the capacitors comprisingthis invention are soldered into an electrical circuit.

Thermosetting cement is used to cement the terminal head to the coatedelectrode recess within the ceramic body of the capacitor. Such a cementmay be epoxy conductive cement which not only has good affinity to metalbut also good aflinity to other metals which are not solderable, butpossess desirable characteristics. The use of this type of cement alsoeliminates displacement of the terminal head in the coated electroderecesses due to intense heat during the soldering operation of circuitassembly or during circuit operation. Thermosetting conductive cementeliminates the possibility of dissolving the silver electrode materialas might result if solder were used in place of this cement.Furthermore, this type of cement possess high strength upon curing andcan be made with proper viscosity without use of a solvent therebyeliminating blistering and produce a by-product upon curing which mayreduce the good electrical characteristics of the capacitor.

The terminals are provided with terminal heads which have spiral groovesor indentations which aid in obtaining good adhesion upon cementing theterminal head in the coated electrode recesses.

The method of manufacture comprising this invention provides acylindrical construction for a ceramic capacitor which reduces a voltagestress at the edges of the electrodes. The concentration of electricalstress is at the edges of the electrodes because the edges of theelectrodes are not in cross relationship to each other as found in thestructures of the conventional disc type capacitors. Since the electrodeedges at the inside edge of each of the recesses have a large dielectricseparation from each other, there is less chance that dielectricbreakdown will occur in a capacitor manufactured under the method ofthis invention.

Another advantage of the method of manufacturing of this invention isthat the coated electrode recesses are self-insulated from theatmosphere and, thus, other electrical circuit components. There is,therefore, no need for exterior insulation and, furthermore, thedisadvantage of dirt and moisture penetration, which degrades theelectrical properties of the capacitor, are eliminated.

'The method of manufacturing of a ceramic capacitor comprising thisinvention provides a dielectric body const-ruction having a longersurface leakage path between the electrodes of the capacitor as comparedwith electrical discs and tubular capacitors and, thus, further reducesthe chance of dielectric breakdown wherein the surfaces of thecapacitors are subjected to the presence of moisture, dirt or otherforeign matter collecting thereon, reducing the capacitors dielectricstrength through the for mation of a conductive path between theelectrodes.

Other objects and advantages appear hereinafter in the followingdescription and claims.

The accompanying drawings show for the purpose of exemplificationwithout limiting the invention or the claims thereto certain practicalembodiments illustrating the principles of this invention wherein:

FIG. 1 shows an exploded and enlarged view of the capacitor comprisingthis invention.

FIG. 2 shows an enlarged axial cross-sectional view of the capacitorcomprising this invention.

FIG. 3 shows the apparatus for producing terminal heads used with thecapacitor comprising this invention.

FIG. 4 shows the apparatus for producing terminal heads before thepressure forming operation.

FIG. shows the apparatus for producing terminal heads after the pressureforming operation.

Referring now to the figures the cylindrical dielectric cylindrical body1 is a ceramic material. At the ends of this cylindrical body 1 there isformed the recesses 2. and 3. The principal purposes of the ceramic bodyas far as capacitor construction is concerned is to insulate the twoelectrodes 4 and 5 of the capacitor from one another and also insulatethem from other circuit components and support them in capacitiverelation to each other. Furthermore, due to the construction of thisceramic body 1 the electrodes are protected from atmospheric elementssuch as moisture, dust and any other foreign material which could affectthe electrical properties possessed by the electrodes 4 and 5.

The cylindrical dielectric body 1 is made first by blending together therequired ceramic powder including metallic oxides in various proportionsdepending on the required capacitance to be obtained, the temperaturecoefficient of capacity required, and the Q value desired after theceramic body has been baked. A temporary binder such as phenolic resinor wax emulsion in water is added to the mixture of ceramic powders inorder to temporarily hold the blended elements in a homogeneous massuntil the baking operation is completed. The temporary binder will burnout completely during the baking operation and, therefore, is used onlyto maintain the form of the ceramic body in its required shape prior tothe high fusion temperatures tion.

The ceramic powders used in this blend can be such metallic oxides suchas barium titanium oxides (BaTiO and titanium oxide (TiO After thebinder is thoroughly mixed in the ceramic powders, the mixture is thendried and granulated to the proper mesh for the molding operation. Theceramic powder granules are then placed in a cylindrical steel mold. Thesteel mold has end punches which have a projection on their face forforming the body and the recesses in the ends of the ceramic body 1. Theend punches are inserted in each end of the mold, and pressure isapplied resulting in the formed ceramic body 1. The ceramic body 1 isthen ejected from the mold and is ready for the baking operation.

The baking operation may be carried out in an electrically heatedfurnace having an oxidizing atmosphere at temperatures between 2100 F.and 2500 F. However, it should be realized that the time and temperatureof this baking operation is dependent upon the size and shape of theparticular ceramic body being processed. After the baking operation theceramic body 1 possesses a vitreous quality and posseses greatmechanical strength.

The electrodes 4 and 5 are then formed inside the recesses of theceramic body. The electrodes 4- and 5 are coated upon the bottom and thecylindrical side walls up to the inside edges 6 and 7 of the recesses 2and 3 respectively. The preferred electrode material is silver becauseof its excellent conductivity ability and also because of the higherQ-value obtainable in the capacitor. Furthermore, silver makes moreintimate contact with the ceramic dielectric.

The silver electrodes are made by coating these surfaces 2 and 3 with acommercial silver paste which consists of pure silver powder, glassfrit, and temporary organic binders. After coating, the parts are bakedat a low temperature to expel any volatile thinners which may blisterthe electrodes reducing their intimate bond with the ceramic dielectricupon heating the ceramic body to a higher temperature. After this bakingby heating at lower temperature, the ceramic body 1 with the silvercoated electrodes is then placed in an oven at a temperature of about1200" F. for approximately twenty minutes. The silver glas frit blendmelts and adheres to the ceramic surface of the recesses 2 and 3 thusforming conductive metallic electrodes 4 and 5 firmly bonded to theceramic.

used in the baking opera- The terminals 8 and 10 consists ofsubstantially solid cylindrical terminal heads 11 and 12. and stems 13and 1 which protrude from the center of one end of the terminal heads 11and 12 respectively. Spiral or circumferential grooves or indentations15 are formed during the coiling process on the perimetral surface ofthe cylindrical heads 11 and 12 which spiral toward the ends of theterminal head. The spiral groove or indentation 15 terminates at 16 inthe center of the head. The terminal heads 11 and 12 may be formed froma single copper wire. One end of this copper wire is upset in a cavitywhich forms the coiled terminal head. Thus the wire stems 13 and 14 areintegral with the terminal heads 11 and 12 respectively. The terminalconnections 8 and 10 may then be tin coated for good solderability whenthe finished capacitor is asembled in a circuit.

FIGS. 3, 4 and 5 show the manner in which the terminal heads 11 and 12are formed. A copper wire such as the stem 13 is placed in the clamp 18with a sufiicient amount of its length projecting as shown at 19 to forma terminal head by compression coiling. An open cylinder die 20 isprovided with a piston 21 at one end. As shown in FIG. 4 the open end ofthe cylinder 20 is placed over the end 19 of the wire 13 and in contactwith the smooth and flat face of the clamp 18. The cylinder 20 and theclamp 1% are held in fixed relationship to each other with suitableclamp means. Pressure is then applied to the piston 21, and as shown inFIG. 5, the piston causes the wire to coil. The spiral grooves 15 aremerely the gaps or grooves formed between adjacent wire turns whencoiled or pressed into a head. Coiling and upsetting the wire 13produces the spiral groove 15.

The terminal heads 11 and 12 are secured within the recesses 2 and 3 tothe silver electrodes 4 and 5 by using a thermosetting conductive cement17 such as epoxy conductive cement. Such a cement is used in theconstruction of this type of capacitor because epoxy resin used in thiscement possesse high strength upon Curing Which i needed for goodmechanical axial terminal anchorage. Furthermore, epoxy cement has greataflinity for metal as well as non-metallic materials which also may begood electrode materials such as colloidal graphite or air dry metallicconductive coatings. Such electrode materials are not solderable whichis not a necessary requirement when epoxy cement is employed.

Epoxy cement is thermosetting and is available in the proper viscositywithout the use of any solvent which otherwise might cause blistering ifthe solvent is not completely removed before the curing process. Alsoother types of cement may produce other unwanted by-products in thecuring process and thereby reduce the good electrical characteristics ofthe capacitor. Silver powder is used as a filler in the thermosettingcement not only because of its high conductivity but also because of theresultant high Q-value in the capacitor. Furthermore, silver has thegood characteristic of resisting oxidation during the curing process andis compatible with the resin.

Epoxy conductive cement makes a very mechanically strong connectionbetween the terminal heads 11 and 12 and the electrodes 4 and 5 in therecesses 2 and 3 and because of the resistance of epoxy resin to hightemperatures such as temperatures up to 700 F., without loss ofmechanically axial support, strength and adhesion with the terminals 4and 5 in the recesses 2 and 3. If solder is used in place ofthermosetting cement, the conducting of heat through the stems 13 and14- and heads 11 and 12 during circuit operation may cause the terminalconnections 8 and 10 to move out of position due to the melting orweakening of the solder. This is avoided by using thermosetting cement.Furthermore, by using thermosetting conductive cement, there is nopossibility of dissolving or otherwise disintegrating the silverelectrode material as might frequently occur if solder were used andwhere special precautions would have to be taken. Thus the electrodes 4and 5 may be made of a thinner silver coating than is normally usedbeing more practical and consequently lower cost in manufacturing thesecapacitors.

The epoxy conductive cement used is a product which consists of silverpowder, epoxy resin and a catalyst. The terminal heads 11 and 12 arecoated with this epoxy conductive cement 17 and then are placed in therecesses 2 and 3 of the ceramic body 1. The spiral grooves orindentation 15 aid in ensuring good adhesion between the terminal heads11 and 12 and the electrodes 4 and 5 respectively. The cemented assemblyas shown in FIG. 2 is then placed in an oven at a relatively lowtemperature for curing the epoxy cement in order to produce mechanicallystrong homogeneous bond between the electrodes 4 and 5 and the terminalheads 11 and 12 respectively.

In order to bring out more clearly the manufacturing process used inmaking the ceramic capacitors comprising this invention the followingexample i given. This example is in no way restrictive as to theapplication of the method comprising this invention but is merely todemonstrate more clearly the method comprising this invention in view ofthe foregoing.

9300 grams of titanium oxide (TiO 350 grams of Zirconium oxide (ZrO 280grams of zinc oxide (ZnO) and 70 grams of bentonite are thoroughlyblended together. A small quantity of phenolic resin is added to act asthe temporary binder. A ceramic body is then formed by using a cylinderwith piston ends having central protrusions to form the recesses, asexplained above. The capacitance may be varied by adjusting the web orcenter portion thickness which depends, then, on the amount of metallicoxides used and pressure applied in the forming operation.

The molded body is then baked for ten hours at a peak temperature of2200 F. The first four hours are required to reach the peak temperature.For the next two hours the molded ceramic body is baked at the constantpeak temperature of 2200 F. The last four hours are required for themolded body to slowly cool down to room temperature. The resultant bakedceramic body has the following Thickness of web or central portionbetween the recesse .030

Shrinkage, based on the ceramic body dimensions before firing, isapproximately 15%.

The recesses 2 and 3 are thereafter coated with silver paste to formelectrodes as fully described above. The ceramic body is heated first toa low temperature to expel any volatile thinners present in the past andthereafter heated to a temperature of 1200 F. for twenty minutes to meltthe paste and form a homogeneous bond between the metallic silverforming the electrodes and the ceramic. Epoxy conductive cement isapplied to the terminal heads which are then placed within the recesses2 and 3. The assembled capacitor is then baked for one hour at atemperature of 350 F. to cure the cement and form a strong conductivebond between the terminal heads 1.1 and 12 and the electrodes 4 and 5respectively.

The resultant capacitor has a capacitance of 6 [.L/Lf. and a temperaturecoefiicient of 750 parts/ lO /deg. C. This capacitor has a Q-value at1.0 me. of approximately 1500.

The embodiment shown in the drawings and described above is given merelyfor the purposes of explanation and illustration without intending tolimit the scope of the claims to the specific details disclosed. Itshould be understood that the description of the preferred forms of theinvention are for the purpose of complying with Section 112, Title 35 ofthe US. Code and that the claims should be construed as broadly as priorart will permit.

I claim:

1. A method for manufacturing cylindrical ceramic capacitors whichconsists of the steps of mixing ceramic powder granules with a temporarybinder, molding the mixture to form a cylindrical ceramic body having anaxial aligned recess in each end thereof, baking the ceramic body tovitrify the same, coating each recess with a silver paste to formelectrodes, baking the ceramic coated body to fuse the coating andproduce homogeneous electrodes bonded on the ceramic body, forming asolid cylindrical terminal head with a spiral groove along the terminalhead perimetral surface by upsetting under compression one end of aterminal wire, applying a thermosetting conductive cement between theterminal head and electrode in each recess, and curing the cement bybaking the assembled capacitor to form a bond between each electrode ineach respective terminal head.

References Cited UNITED STATES PATENTS 2,527,373 10/1950 Parson 3l7-2423,052,573 9/1962 Dumesnil 294205 3,221,387 11/1965 Weller et al 2925.423,274,467 10/1966 Graf 3l7258 FOREIGN PATENTS 867,929 2/ 1953 Germany.

JOHN F. CAMPBELL, Primary Examiner.

RICHARD B. LAZARUS, Assistant Examiner.

